"Manuka honey differed from the profiles of other antibacterial agents, indicating a unique mode of action and its potential value as a novel antimicrobial agent," they say.

Honey has traditionally been used as an antibacterial agent, says microbiologist Associate Professor Dee Carter from the University of Sydney.

She says one reason for this is because it contains glucose oxidase that generates small amounts of hydrogen peroxidise in the presence of water, which is toxic to microorganisms.

Honey that comes from the bees foraging on the New Zealand manuka plant (Leptospermum scoparium), however, can have very high levels of antibacterial activity, even if the hydrogen peroxide in it is broken down.

Probing manuka honey's secrets

Carter and colleagues set out to find more about the unknown antibacterial compound that was often found to be present in manuka honey.

The researchers treated cells of the wound pathogen Staphylococcus aureus with different types of honey.

As their control they used a leptospermum honey that did not appear to have this extra antibacterial agent and neutralised the hydrogen peroxide in it.

They then compared the effect of this honey on the bacterial cells, to the effect of full-strength antibacterial manuka honey containing the unknown compound.

After half an hour they extracted proteins from the staphylococcus cells to see how they were responding to the honey treatment.

The researchers report treatment with manuka honey resulted in a significant decrease in the bacterial cell growth rate.

It also resulted in downregulation of ten proteins, including those involved in creating cell proteins, and upregulation of two proteins, including a stress-related protein, they say.

"On the whole it is suppressing cellular function," says Carter.

"The honey is interacting with the cell in some way that's causing the cell to know that it's in trouble and to start shutting down certain things that it doesn't really need at the time."

Complication

In the middle of the study being carried out by Carter and colleagues, European researchers discovered that manuka honey contained an antibacterial agent called methyl- glyoxal or MGO.

"That complicated our analysis a bit because if we had known about that we would have used that as a control as well," says Carter.

She says some studies have suggested that the level of MGO in manuka honey are not high enough in its own right to give the degree of killing that's seen.

Carter says it would have been good to tease out the exact role of MGO in their study and explore whether there is some other factor that also gives manuka honey its antibacterial activity.

She says another mystery is that MGO binds to protein and DNA, and is very toxic to human cells when it's not contained in honey. When it is contained in honey it will not damage human cells, but still kill bacterial cells.

Carter and colleagues are now looking at the impact of MGO on the death of bacterial cells.

Not all manuka honey is equally active, says Carter, and it is important to measure the honey's ability to kill bacterial cells directly rather than just measuring MGO levels.